It sounds like your collaborator hasn't accepted your sharing request or set sharing permissions. Make sure they go to the collaboration page and accept your request. Here's the link to our collaboration tutorial video:

There is a full explanation in the tutorial video I posted above. They will need to go to the collaboration page in the "tools" menu (http://www.toodledo.com/share.php) and then accept your request there. Then, they can also add you as a collaborator and edit share settings by clicking on your profile.